The present invention relates to a magnetoresistive (MR) sensor. More specifically, the present invention relates to an MR sensor utilizing side shields to reduce side reading from adjacent tracks.
Magnetoresistive (MR) sensors utilize an MR stack to read magnetically encoded information from a magnetic medium, such as a disc, by detecting magnetic flux stored on the magnetic medium. Each disc is typically organized into a large number of tracks. Each track is divided into bit fields, which are magnetized in one direction or the other representing stored information on a disc. The storage capacity of a disc is related to the size and is areal density of the disc surface. Areal density per square inch on the disc surface is given by the product of BPI (bits per inch) and TPI (tracks per inch). The objective is to place as many tracks and bits as possible on the surface of the disc in order to maximize the total bit capacity.
When an MR sensor reads a magnetic medium, the MR sensor must be positioned close enough to the medium that magnetic fields extending from the medium will go through the MR sensor. At a particular point in time, the MR sensor is positioned to read a particular bit within a particular track. Ideally only the magnetic field from one bit at a time will be read. However, magnetic flux emanates from adjacent bits within the particular track and from bits within adjacent tracks, which the MR sensor is not intended to read at this particular point in time. As areal density increases, the distance between the adjacent tracks decreases allowing the magnetic flux to penetrate the MR sensor thereby receiving magnetic field signals (sidereading) from adjacent tracks.
The focus of the art has been to increase linear bit density, which is related to the stack height or shield-to-shield spacing of the MR sensor. Many MR sensor structures include top and bottom shields in order to isolate the bit currently being read from the adjacent bits within that track, but lack the ability to isolate the MR stack from the magnetic flux of adjacent tracks.
As areal densities increase and sensor dimensions decrease, the issue of sidereading becomes a greater limitation to track densification. High linear density (BPI) is possible because the MR stack is shielded top and bottom by magnetically permeable materials. MR sensors without shielding on the sides facing adjacent tracks causes the magnetic sensing width of the sensor to potentially become greater than two times the physical width of the MR stack. Without side shields, signals from adjacent tracks can be detected by the MR stack, which results in signal noise. To compensate for the magnetic sensing width being greater than the physical width of the MR stack, the physical width of the stack has been made narrower and consequently the sensor has incurred amplitude and process control penalties.
In order to combat the wider magnetic sensing width of the MR stack and problems of track misregistration of the reader head, guard bands or empty spaces may be provided on two sides of each reading track thereby reducing the possibility of the sensor reading information from adjacent tracks. However, the inclusion of these empty spaces reduces track density and consequently areal density of the disc. Continued increases of disc capacity require improvement in MR sensor structure, for example by eliminating the need for or by reducing the size of empty spaces between reading tracks by incorporation of effective side shielding into the reader head design.
The basic structure of an MR sensor includes a bottom shield, an MR stack, hard bias elements, contacts, an insulator layer, and a top shield. The MR stack is a stack of layers of materials formed onto a bottom shield. Variation in magnetic flux passing through an MR stack causes a detectable change in the electrical properties or signal. The configuration of the layers of an MR stack varies by type, one example being a spin valve which includes from the bottom shield, an antiferromagnetic layer, a pinned layer, a spacer layer and, at the top of the stack, a free layer. Hard bias elements are located on either side of the MR stack and define the active portion of the sensor. Hard bias elements are commonly ferromagnetic materials with high coercivity such as CoPt and CoCrPt. Hard bias elements act to stabilize the response of the sensor and set the quiescent state of the sensor. Contacts are typically formed on top of the hard bias elements. Materials commonly used in contacts are electrically conductive, non-magnetic materials including Ta, Ti/W and Au. One contact is laid on each side of the MR stack and typically contacts the edges of the top layer of MR stack in CIP (current in plane) sensors. In this manner the contacts can provide a path for electrical current to and across the MR stack. Contacts and the MR stack are typically separated from the top shield by an insulator layer.
In order to effectively constrain the sensing width of an MR sensor, shielding materials could be introduced on the sides of the sensor, for example by forming shields from a NiFe alloy or Permalloy as is used for the top and bottom shields. However, this structure does not allow for inclusion of domain control structures, which are typically located to either side of the MR stack. Examples of domain control structures include: hard bias elements or alternating layers of magnetically soft materials coupled with antiferromagnetic materials. Biasing is critical to the proper operation of the sensor. Hard bias elements prevent formation of closure domains at the ends of the MR stack, reduce Barkhausen noise and hysteresis in the magnetoresistive response of the sensor. Consequently, the displacement of domain control structures in favor of side shields is not an acceptable solution.
Additionally, Permalloy is unsuitable as a material for side shielding an MR sensor. Permalloy has an anisotropic magnetoresistive signal large enough to interfere with the magnetoresistive signal from the MR stack. Second, the placement of Permalloy, or other soft magnetic material, adjacent to a hard bias element would impair the function of the MR sensor because the Permalloy would magnetically exchange couple to the hard bias element, thereby effectively reducing the coercivity of the hard bias element. Altering the magnetic properties of the hard bias would negatively affect the ability of the hard bias to control the domain structure of the MR stack, consequently harming sensor performance. There is a continuing need in the art for an MR sensor design that incorporates side shielding without otherwise compromising the domain control structures or sensor performance.
Another example to provide some shielding for an MR sensor in the track width direction, is to replace the permanent magnet bias with alternating layers of ferromagnetic (FM) and antiferromagnetic (AFM) materials. This structure has the disadvantage of requiring a magnetic anneal to activate the AFM whereby the applied field is perpendicular to the pinning direction. In other words, the AFM layer(s) must have unidirectional anisotropy to pin the FM layer(s) in the proper orientation for successful operation of the sensor. This requirement decreases the reliability of sensors of this design while increasing the difficulty and expense of manufacturing. Therefore, there is a continuing need for the development of a MR sensor with effective side shielding that can be reliably manufactured without sacrificing overall sensitivity and response of the sensor.